Polypropylene resin expanded particles and method for producing same

Abstract

To provide polypropylene-based resin foam particles which have a wide range of a moldable molding pressure and are excellent in releasability.SOLUTION: Polypropylene-based resin foam particles have a foam core layer composed of a polypropylene-based resin (C), and a coating layer composed of a polypropylene-based resin (S), wherein a melting point of the polypropylene-based resin (C) is 125°C or higher and 170°C or lower, and the resin foam particles satisfy the following (1) to (4). (1) The polypropylene-based resin (S) has a melting peak at a low temperature side and a melting peak at a high temperature side. (2) A top temperature of the melting peak on the low temperature side of the polypropylene-based resin (S) is lower than a melting point of the polypropylene-based resin (C). (3) A top temperature of the melting peak on the high temperature side of the polypropylene-based resin (S) is equal to or higher than 138°C. (4) A difference between the melting point of the polypropylene-based resin (C) and the top temperature of the melting peak on the high temperature side of the polypropylene-based resin (S) is -10°C or more and 15°C or less.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to an expanded polypropylene resin particle and a method for producing the same. [Background technology]

[0002] Expanded bead moldings obtained by molding expanded polypropylene resin beads in a mold have excellent chemical resistance, impact resistance, compression strain recovery, etc., and are therefore used in a wide range of fields, such as as impact absorbing materials, heat insulating materials, various packaging materials, food containers, packaging or cushioning materials for electric and electronic parts, automobile bumpers and interior parts, building materials such as residential insulation materials, and miscellaneous goods.

[0003] The polypropylene resin expanded bead molding is produced, for example, by filling a mold with polypropylene resin expanded beads and heating the expanded beads with a heating medium such as steam to secondary expand the expanded beads and melt their surfaces to fuse them together and mold them into a desired shape by an in-mold molding method. In the in-mold molding of the polypropylene resin expanded beads, the heating temperature by the heating medium is set mainly taking into consideration the melting point of the resin constituting the foam layer.

[0004] Since polypropylene-based resin expanded beads have difficulty in molding due to the crystallinity and heat resistance of the propylene-based resin, studies are being conducted to improve the molding processability. For example, Patent Document 1 describes a method for producing polypropylene-based resin expanded beads, which comprises a core layer formed of a polypropylene-based resin and an outer layer formed of a polypropylene-based resin, the melting points of the polypropylene-based resin of the outer layer and the polypropylene-based resin of the core layer having a specific relationship, and the thickness of the outer layer is a specific value or less, and is impregnated with a foaming agent to be expanded, and the polypropylene-based resin expanded beads obtained by this production method have the effect of excellent fusion properties between the expanded beads at a low molding heating temperature (molding pressure).

[0005] However, the polypropylene resin expanded beads described in Patent Document 1 may have poor releasability depending on the molding conditions and the type of base resin. In particular, when expanded beads with low apparent density are used or when the molding heating temperature is high, the releasability may be poor. Therefore, from the viewpoint of further increasing the productivity of expanded bead moldings, expanded beads with good releasability over a wide range of molding heating temperatures are required. In order to improve the releasability of expanded beads, for example, Patent Documents 2 and 3 disclose molding devices in which comb teeth are provided on a movable member in a mold or molding space. In addition, for example, Patent Document 4 discloses a method for producing expanded bead moldings using a mold in which pores or slits having a cross-sectional shape whose opening area expands toward the surface in contact with the expanded beads are formed on at least one side of the inner wall surface of the mold. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2004-68016 A [Patent Document 2] JP 2001-328135 A [Patent Document 3] JP 2002-172642 A [Patent Document 4] JP 2015-214110 A Summary of the Invention [Problem to be solved by the invention]

[0007] However, Patent Documents 2 to 4 require dedicated equipment, and even when a general-purpose equipment is used, it is desirable to realize expanded polypropylene resin beads that can be molded into expanded bead moldings over a wide range of heating temperatures and have excellent release properties.

[0008] Therefore, an object of the present invention is to provide expanded polypropylene resin beads which can be molded over a wide range of heating temperatures and have excellent mold releasability, and a method for producing the same. [Means for solving the problem]

[0009] The present inventors have found that the above problems can be solved by adopting the configuration described below, and have completed the present invention. That is, the present invention is as follows. <1> The expanded polypropylene resin particles have an expanded core layer made of a polypropylene resin (C) and a coating layer made of a polypropylene resin (S), the melting point Tmc of the polypropylene resin (C) being 125°C or higher and 170°C or lower, and satisfying the following (1) to (4): (1) The polypropylene-based resin (S) has a melting peak on the low temperature side and a melting peak on the high temperature side in a second DSC curve obtained by heat flux differential scanning calorimetry, in which the resin is heated from 30°C to 200°C at a heating rate of 10°C / min, then cooled from 200°C to 30°C at a cooling rate of 10°C / min, and then heated again at a heating rate of 10°C / min. (2) The apex temperature (Tmsl) of the melting peak on the low temperature side of the polypropylene-based resin (S) is lower than the melting point (Tmc) of the polypropylene-based resin (C). (3) The polypropylene resin (S) has a melting peak on the high temperature side (Tmsh) of 138°C or higher. (4) The difference [Tmc-Tmsh] between the melting point Tmc of the polypropylene resin (C) and the apex temperature Tmsh of the melting peak on the higher temperature side of the polypropylene resin (S) is -10°C or more and 15°C or less. <2> The polypropylene resin (S) has a melting peak apex temperature (Tmsl) on the low temperature side of 125° C. or more and 135° C. or less. <1> 2. The expanded polypropylene resin particles according to claim 1 . <3> The difference [Tmc-Tmsl] between the melting point Tmc of the polypropylene-based resin (C) and the apex temperature Tmsl of the melting peak on the low temperature side of the polypropylene-based resin (S) is more than 0°C and 25°C or less. <1> or <2> 2. The expanded polypropylene resin particles according to claim 1 . <4> the ratio [ΔHh / ΔHt] of the heat of fusion (ΔHh) of the melting peak on the high temperature side of the polypropylene resin (S) to the total heat of fusion (ΔHt) of the polypropylene resin (S) is 0.35 or more and 0.80 or less; <1> ~ <3> 10. The expanded polypropylene resin particles according to claim 9 . <5> The difference [Tmsh-Tmsl] between the apex temperature Tmsh of the melting peak on the higher temperature side and the apex temperature Tmsl of the melting peak on the lower temperature side of the polypropylene-based resin (S) is 10°C or more and 20°C or less. <1> ~ <4> 10. The expanded polypropylene resin particles according to claim 9 . <6> The melting point Tmc of the polypropylene resin (C) is 130°C or more and 150°C or less. <1> ~ <5> 10. The expanded polypropylene resin particles according to claim 9 . <7> The polypropylene-based resin (S) is a propylene-based copolymer containing a propylene component as a main component and an ethylene component and a butene component. <1> ~ <6> 10. The expanded polypropylene resin particles according to claim 9 . <8> The bulk density of the polypropylene resin foam particles is 10 kg / m 3 More than 30kg / m 3 Below is the <1> ~ <6> 10. The expanded polypropylene resin particles according to claim 9 . <9> A method for producing expanded polypropylene-based resin beads, comprising the steps of: granulating multilayer resin particles having a core layer made of a polypropylene-based resin (C) and a coating layer made of a polypropylene-based resin (S) that coats the core layer; dispersing the multilayer resin particles in a dispersion medium in a sealed container; impregnating the multilayer resin particles dispersed in the dispersion medium with a foaming agent; and expanding the multilayer resin particles containing the foaming agent to produce expanded beads, wherein the polypropylene-based resin (C) has a melting point Tmc of 125°C or higher and 170°C or lower, and the following (1) to (4) are satisfied: (1) The polypropylene-based resin (S) has a melting peak on the low temperature side and a melting peak on the high temperature side in a second DSC curve obtained by heat flux differential scanning calorimetry, in which the resin is heated from 30°C to 200°C at a heating rate of 10°C / min, then cooled from 200°C to 30°C at a cooling rate of 10°C / min, and then heated again at a heating rate of 10°C / min. (2) The apex temperature (Tmsl) of the melting peak on the low temperature side of the polypropylene-based resin (S) is lower than the melting point (Tmc) of the polypropylene-based resin (C). (3) The polypropylene resin (S) has a melting peak on the high temperature side (Tmsh) of 138°C or higher. (4) The difference [Tmc-Tmsh] between the melting point Tmc of the polypropylene resin (C) and the apex temperature Tmsh of the melting peak on the higher temperature side of the polypropylene resin (S) is -10°C or more and 15°C or less. Effect of the Invention

[0010] According to the present invention, it is possible to provide expanded polypropylene resin beads which can be molded over a wide range of molding heating temperatures and which have excellent mold releasability over the entire range. [Brief description of the drawings]

[0011] [Figure 1] FIG. 2 is a diagram showing a second DSC curve of the polypropylene resin of the coating layer used in Example 1. [Diagram 2] FIG. 13 is a diagram for explaining how to obtain the high-temperature peak of an expanded bead. [Diagram 3] FIG. 2 is a diagram showing a second DSC curve of the polypropylene resin of the coating layer used in Comparative Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] [Polypropylene resin foam particles] The expanded polypropylene resin particles of the present invention (hereinafter also simply referred to as expanded polypropylene resin particles or expanded particles) are expanded polypropylene resin particles having an expanded core layer (hereinafter also simply referred to as expanded core layer) composed of a polypropylene resin (C) and a coating layer (hereinafter also simply referred to as coating layer) composed of a polypropylene resin (S), in which the melting point Tmc of the polypropylene resin (C) is 125°C or more and 170°C or less and satisfies the following (1) to (4). (1) The polypropylene resin (S) has a melting peak on the low temperature side and a melting peak on the high temperature side in a second DSC curve obtained by heat flux differential scanning calorimetry, in which the resin is heated from 30°C to 200°C at a heating rate of 10°C / min, then cooled from 200°C to 30°C at a cooling rate of 10°C / min, and then heated again at a heating rate of 10°C / min. (2) The apex temperature (Tmsl) of the melting peak on the low temperature side of the polypropylene-based resin (S) is lower than the melting point (Tmc) of the polypropylene-based resin (C). (3) The polypropylene resin (S) has a melting peak on the high temperature side (Tmsh) of 138°C or higher. (4) The difference [Tmc-Tmsh] between the melting point Tmc of the polypropylene-based resin (C) and the apex temperature Tmsh of the melting peak on the higher temperature side of the polypropylene-based resin (S) is -10°C or more and 15°C or less.

[0013] The expanded polypropylene resin beads of the present invention are expanded beads with a multilayer structure in which a specific relationship between a polypropylene resin (C) and a polypropylene resin (S) serves as an expanded core layer and a coating layer, respectively, and therefore have a wide range of molding pressures, i.e., excellent moldability and releasability. Specifically, it has been found that in order to achieve both moldability and releasability, it is effective to use expanded polypropylene resin particles that satisfy the above requirements (1) to (4). Conventionally, in order to improve fusibility, expanded beads provided with a coating layer made of a polypropylene resin having a melting point lower than that of the polypropylene resin constituting the foamed core layer tended to have impaired releasability. In particular, under conditions of high molding heating temperature, the resin of the coating layer softens and tends to adhere to the molding die, so that the releasability tends to be significantly impaired. In addition, even when expanded beads with a low apparent density are used, the releasability tends to be significantly impaired. The present invention has found that polypropylene resin expanded beads can have excellent fusibility and can achieve both moldability and releasability by satisfying the above (1) to (4).

[0014] <Coating layer> The coating layer of the expanded polypropylene resin particles coats the expanded core layer. The coating layer preferably covers almost the entire outer surface of the expanded core layer, and may completely cover the entire outer surface of the expanded core layer. Specifically, the coating layer preferably covers 50% or more of the surface area of ​​the core layer, more preferably 70% or more, and may cover substantially 100% or 90% or less.

[0015] From the viewpoint of the appearance of the polypropylene resin expanded bead molding (hereinafter also referred to as polypropylene resin expanded bead molding or expanded bead molding) obtained by using the polypropylene resin expanded beads of the present invention, the coating layer is preferably in a non-foamed state or substantially non-foamed state, more preferably in a non-foamed state. The non-foamed state or substantially non-foamed state means that there is almost no bubble structure. It also includes a state in which bubbles once formed are broken and disappear. The coating layer may be in a slightly foamed state as long as it does not impair the intended effect of the present invention.

[0016] (Polypropylene resin (S)) The coating layer of the polypropylene resin expanded particles is composed of a polypropylene resin (S). In this specification, the polypropylene resin refers to a polymer having a content of structural units derived from propylene of 50% by mass or more. Examples of the polypropylene resin include a propylene homopolymer, a propylene copolymer, or a mixture thereof. The polypropylene resin may contain other polymers such as resins other than polypropylene and elastomers, and the content is preferably 20% by mass or less, more preferably 15% by mass or less, even more preferably 10% by mass or less, and particularly preferably 5% by mass or less.

[0017] <(1) Double Peak> The polypropylene resin (S) has a low-temperature melting peak and a high-temperature melting peak in a DSC curve obtained by heating a conditioned test piece from 30°C to 200°C at a heating rate of 10°C / min, based on JIS K 7121:1987, using "(2) A case where the melting temperature is measured after performing a certain heat treatment" as the conditioning. Specifically, a resin conditioned at 23°C and 50% RH for 24 hours or more is used as a test piece, and the resin is heated from 30°C to 200°C at a heating rate of 10°C / min by heat flux differential scanning calorimetry, then cooled from 200°C to 30°C at a cooling rate of 10°C / min, and heated again at a heating rate of 10°C / min, and the second DSC curve obtained has a low-temperature melting peak and a high-temperature melting peak. Here, having a low-temperature melting peak and a high-temperature melting peak means that the low-temperature melting peak and the high-temperature melting peak have different peak apex temperatures (melting points) and each has a heat of fusion of 10 J / g or more. The method for measuring the heat of fusion will be described later. The low-temperature melting peak and the high-temperature melting peak may partially overlap. In this specification, the low-temperature melting peak and the high-temperature melting peak in the second DSC curve of the polypropylene resin (S) may be collectively referred to as a double peak. In addition, when three or more melting peaks appear in the second DSC curve of the polypropylene resin (S), the melting peak having the lowest peak temperature and a heat of fusion of 10 J / g or more is taken as the heat of fusion on the low temperature side, and the melting peak having the highest peak temperature and a heat of fusion of 10 J / g or more is taken as the melting peak on the high temperature side. If the polypropylene-based resin (S) does not have a melting peak on the low temperature side and a melting peak on the high temperature side (double peak) in the second DSC curve, but has only a single melting peak, there is a risk that the polypropylene-based resin foamed beads will not be able to have both good moldability and good releasability.

[0018] (2) The peak temperature of the melting peak on the low-temperature side, Tmsl The apex temperature Tmsl of the melting peak on the low temperature side of the polypropylene resin (S) is lower than the melting point Tmc of the polypropylene resin (C) described later. When Tmsl is lower than Tmc, the fusion property is excellent even at a low molding heating temperature, and moldability is improved. From the same viewpoint, Tmsl is preferably 135°C or less, more preferably 132°C or less, and even more preferably 130°C or less. On the other hand, from the viewpoint of further improving the mold releasability (hereinafter also simply referred to as mold releasability) of the expanded polypropylene resin particles, Tmsl is preferably 125°C or more, more preferably 126°C or more, and even more preferably 127°C or more.

[0019] (3) The top temperature of the melting peak on the high temperature side, Tmsh The apex temperature Tmsh of the melting peak on the high temperature side of the polypropylene resin (S) is 138°C or higher. If Tmsh is less than 138°C, the releasability of the expanded beads may be impaired, particularly under conditions of high molding heating temperature. From this viewpoint, Tmsh is preferably 139°C or higher, more preferably 140°C or higher, and even more preferably 141°C or higher. From the viewpoint of further improving moldability, Tmsh is preferably 170°C or lower, more preferably 160°C or lower, even more preferably 155°C or lower, even more preferably 150°C or lower, and even more preferably 145°C or lower.

[0020] (4) Difference between melting points Tmc and Tmsh of polypropylene resin (C) The difference [Tmc-Tmsh] between the melting point Tmc of the polypropylene resin (C) described later and the apex temperature Tmsh of the melting peak on the high temperature side of the polypropylene resin (S) is -10°C or more and 15°C or less. When the difference [Tmc-Tmsh] exceeds 15°C, the releasability of the expanded beads may be impaired when a resin with a relatively high melting point is used. From this viewpoint, the difference [Tmc-Tmsh] is preferably 12°C or less, more preferably 10°C or less, and even more preferably 7°C or less. In addition, when the difference [Tmc-Tmsh] is less than -10°C, there is a risk that both moldability and releasability cannot be achieved. From the viewpoint of improving releasability while suppressing a decrease in moldability, the difference [Tmc-Tmsh] is preferably -8°C or more, and even more preferably -5°C or more.

[0021] <Difference between melting points Tmc and Tmsl of polypropylene resin (C)> The difference [Tmc-Tmsl] between the melting point Tmc of the polypropylene resin (C) described later and the apex temperature Tmsl of the melting peak on the low temperature side of the polypropylene resin (S) exceeds 0°C, as described above, from the viewpoint of moldability. From the viewpoint of further improving the moldability of the expanded beads, the difference [Tmc-Tmsl] is more preferably 5°C or more, and even more preferably 10°C or more. On the other hand, from the viewpoint of further improving the releasability and from the viewpoint of suppressing peeling between the foamed core layer and the coating layer, the difference [Tmc-Tmsl] is preferably 25°C or less, more preferably 22°C or less, and even more preferably 20°C or less.

[0022] <Difference between Tmsh and Tmsl> From the viewpoint of achieving a good balance between moldability and releasability, the difference [Tmsh-Tmsl] between the apex temperature Tmsh of the melting peak on the higher temperature side and the apex temperature Tmsl of the melting peak on the lower temperature side of the polypropylene resin (S) is preferably 10°C or more, more preferably 13°C or more, and is preferably 20°C or less, more preferably 17°C or less, and even more preferably 15°C or less.

[0023] <Ratio of the heat of fusion ΔHh of the high-temperature melting peak to the total heat of fusion ΔHt> From the viewpoint of achieving a good balance between moldability and releasability, the ratio of the heat of fusion ΔHh of the high temperature melting peak to the total heat of fusion ΔHt of the polypropylene resin (S) [ΔHh / ΔHt] is preferably 0.35 or more, more preferably 0.38 or more, even more preferably 0.40 or more, and is preferably 0.80 or less, more preferably 0.70 or less, even more preferably 0.60 or less, and even more preferably 0.50 or less.

[0024] The heat of fusion ΔHl of the low-temperature melting peak, the heat of fusion ΔHh of the high-temperature melting peak, and the total heat of fusion ΔHt of the polypropylene resin (S) can be calculated as follows for the DSC curve shown in FIG. 1. The point on the DSC curve at a temperature of 80° C. is taken as α, and the point on the DSC curve corresponding to the melting end temperature is taken as β. In addition, the heat of fusion P l and the melting peak P h From point γ on the DSC curve, which is in the valley between points α and β, draw a straight line parallel to the vertical axis of the graph, and the point where it intersects with the straight line (α-β) connecting points α and β is called δ. Note that the melting peak P l and the melting peak P h If the point γ on the DSC curve that corresponds to the valley between the temperature and the temperature is not clear, the position of the point γ can be determined by referring to the differential curve (DDSC) of the DSC near the temperature. The point γ indicates 0 in the DDSC. Melting peak P at low temperature l The area (A) of the melting peak P l The heat of fusion ΔHl of the low-temperature melting peak P l The DSC curve showing the above-mentioned change in the molecular weight is expressed as the area surrounded by the line segments (α-δ) and (γ-δ). Melting peak P on the high temperature side h The area (B) of the melting peak P h The heat of fusion of ΔHh is the melting peak P h The area is calculated as the area enclosed by the DSC curve showing the above, the line segment (δ-β) and the line segment (γ-δ). The total heat of fusion ΔHt is the melting peak P l The area (A) of the melting peak on the high temperature side P h In FIG. 1, this is the area surrounded by the straight line (α-β) and the DSC curve in the section between point α and point β.

[0025] ≪ΔHl≫ The heat of fusion ΔHl of the melting peak on the low temperature side of the polypropylene resin (S) is preferably 25 J / g or more, more preferably 30 J / g or more, and even more preferably 35 J / g or more from the viewpoint of further improving moldability, and is preferably 50 J / g or less, more preferably 45 J / g or less, and even more preferably 43 J / g or less from the viewpoint of more reliably ensuring releasability.

[0026] ≪ΔHh≫ The heat of fusion ΔHh of the melting peak on the high temperature side of the polypropylene resin (S) is preferably 15 J / g or more, more preferably 21 J / g or more, even more preferably 26 J / g or more, and even more preferably 30 J / g or more from the viewpoint of further improving releasability, and is preferably 45 J / g or less, more preferably 40 J / g or less, and even more preferably 35 J / g or less from the viewpoint of more reliably ensuring moldability.

[0027] ≪ΔHt≫ From the viewpoint of achieving a good balance between moldability and releasability, the total heat of fusion ΔHt of the polypropylene-based resin (S) is preferably 57 J / g or more, more preferably 65 J / g or more, even more preferably 70 J / g or more, and is preferably 85 J / g or less, more preferably 80 J / g or less, even more preferably 75 J / g or less.

[0028] <Melt Flow Rate (MFR)> From the viewpoints of ease of forming the coating layer and suppressing peeling from the foamed core layer, the MFR of the polypropylene resin (S) is preferably 2 g / 10 min or more, more preferably 4 g / 10 min or more, even more preferably 5 g / 10 min or more, and is preferably 10 g / 10 min or less, more preferably 9 g / 10 min or less, even more preferably 8 g / 10 min or less. The MFR of polypropylene resin (S) is measured in accordance with JIS K7210-1:2014 at a temperature of 230°C and a load of 2.16 kg.

[0029] The polypropylene resin (S) is not particularly limited as long as it satisfies the above (1) to (3) and also satisfies the above relationship (4) with the melting point Tmc of the polypropylene resin (C). However, it is preferable that the polypropylene resin (S) is a propylene copolymer containing a propylene component as a main component and an ethylene component and a butene component.

[0030] When the polypropylene resin (S) is a propylene-based copolymer containing a propylene component as a main component and an ethylene component and a butene component, the ethylene component content in the copolymer is preferably 3.0% by mass or more and 6.0% by mass or less, more preferably 3.5% by mass or more and 5.5% by mass or less, and even more preferably 4.0% by mass or more and 5.0% by mass or less. The butene component content in the copolymer is preferably 5.0% by mass or more and 18% by mass or less, more preferably 7.5% by mass or more and 15% by mass or less, and even more preferably 9.0% by mass or more and 12% by mass or less. However, the total of the propylene component, the ethylene component, and the butene component is 100% by mass. The content of the monomer components in the copolymer can be determined by IR spectrum measurement. A propylene-based copolymer having the ethylene component and the butene component content within the above range is likely to have a crystal structure that satisfies the above (1) to (4) and has a low-temperature melting peak and a high-temperature melting peak. From the same viewpoint, the ratio of the butene component content to the ethylene component content [butene component content / ethylene component content] is preferably 1.5 or more, more preferably 1.8 or more, and even more preferably 2.0 or more, and is preferably 5.0 or less, more preferably 4.0 or less, and even more preferably 3.0 or less. In the present invention, examples of resins that can be used as the polypropylene-based resin (S) include FL7540L and FL7320L manufactured by TPC Corporation.

[0031] The polypropylene resin (S) may contain additives within the range that does not impair the intended effects of the present invention.

[0032] <Foam core layer> The expanded core layer of the polypropylene resin expanded beads is composed of a polypropylene resin (C). From the viewpoint of improving the moldability of the expanded beads and the rigidity of the molded body, the closed cell ratio of the expanded beads is preferably 80% or more, more preferably 90% or more, and even more preferably 95% or more. The closed cell ratio of the expanded beads can be measured using an air comparison type specific gravity meter based on ASTM-D2856-70 Procedure C.

[0033] (Polypropylene resin (C)) <Melting point Tmc> The melting point Tmc of the polypropylene resin (C) is 125°C or more, preferably 130°C or more, more preferably 135°C or more, even more preferably 140°C or more, even more preferably 145°C or more, and 170°C or less, preferably 165°C or less, more preferably 160°C or less, even more preferably 155°C or less, and even more preferably 150°C or less. According to the present invention, by using the polypropylene resin (S) as a coating layer, it is possible to achieve both moldability and releasability of the polypropylene resin expanded beads using the polypropylene resin (C) having the above-mentioned wide range of melting point Tmc. Conventionally, in the case of expanded beads using a polypropylene resin having a particularly high melting point as a base resin, when a coating layer is provided to improve fusion, it has been difficult to make the releasability good over the entire moldable molding heating temperature range. According to the present invention, even when a polypropylene resin having a relatively high melting point is used as the foamed core layer, it is possible to achieve both moldability and releasability. The melting point Tmc of the polypropylene resin (C) is determined based on JIS K7121:1987 by adjusting the condition of the test piece in the manner described in "(2) Measuring the melting temperature after a certain heat treatment" and by raising the temperature of the adjusted test piece from 30°C to 200°C at a heating rate of 10°C / min to obtain a DSC curve, and is determined as the apex temperature of the melting peak associated with the melting of the resin on the DSC curve. When multiple melting peaks appear on the DSC curve, the apex temperature of the melting peak with the largest area is determined as the melting point. In this case, the area of ​​each melting peak can be determined by the same method as that for the heat of fusion of the polypropylene resin (S) described above.

[0034] <Flexural modulus> From the viewpoint of achieving both moldability and releasability while maintaining the mechanical strength of the expanded bead molding, the flexural modulus of the polypropylene resin (C) is preferably 500 MPa or more, more preferably 600 MPa or more, even more preferably 700 MPa or more, still more preferably 800 MPa or more, and is preferably 1600 MPa or less, more preferably 1300 MPa or less, and even more preferably 1100 MPa or less. The flexural modulus of the polypropylene resin can be determined based on JIS K7171:2016.

[0035] <MFR> From the viewpoint of foamability, the MFR of the polypropylene resin (C) is preferably 2 g / 10 min or more, more preferably 4 g / 10 min or more, even more preferably 5 g / 10 min or more, and is preferably 15 g / 10 min or less, more preferably 12 g / 10 min or less, even more preferably 10 g / 10 min or less. The MFR of the polypropylene resin (C) is a value measured at 230° C. and under a load of 2.16 kg by the same method as for the MFR of the polypropylene resin (S).

[0036] The polypropylene-based resin (C) is not particularly limited as long as it has a melting point Tmc within the above range, satisfies the above relationship (2) with the top temperature Tmsl of the melting peak on the low temperature side of the polypropylene-based resin (S), and satisfies the above relationship (4) with the top temperature Tmsh of the melting peak on the high temperature side of the polypropylene-based resin (S), but is preferably a mixture of a polypropylene homopolymer and a propylene-based copolymer or a polypropylene-based copolymer.

[0037] The polypropylene-based copolymer is preferably an ethylene-propylene copolymer, an ethylene-propylene-butene copolymer, or a mixture of these copolymers with a polypropylene homopolymer, more preferably an ethylene-propylene copolymer or an ethylene-propylene-butene copolymer, and even more preferably an ethylene-propylene copolymer.

[0038] The polypropylene resin (C) may contain other polymers such as resins other than the propylene homopolymer or propylene copolymer or elastomers, within the range that does not impair the intended effects of the present invention. The content of the other polymers in the polypropylene resin (C) is preferably 20% by mass or less, more preferably 10% by mass or less, even more preferably 5% by mass or less, still more preferably 3% by mass or less, and most preferably 0% by mass. In other words, the polypropylene resin (C) is preferably composed of only polypropylene resin as a polymer.

[0039] The foamed core layer may contain additives such as cell regulators, flame retardants, flame retardant auxiliaries, cell nucleating agents, plasticizers, antistatic agents, antioxidants, ultraviolet inhibitors, light stabilizers, conductive fillers, and antibacterial agents, as required.

[0040] The mass ratio (ratio by mass%) of the resin constituting the foamed core layer to the resin constituting the coating layer is preferably 99.5:0.5 to 80:20, more preferably 99:1 to 85:15, and even more preferably 97:3 to 90:10, from the viewpoint of improving moldability and releasability while maintaining the rigidity of the molded article. The mass ratio is represented by the resin constituting the foamed core layer:the resin constituting the coating layer.

[0041] [Physical properties of polypropylene resin foam particles] <Bulk density> The bulk density of the polypropylene resin foam particles is preferably 10 kg / m from the viewpoint of achieving a good balance between the mechanical strength and the light weight of the foamed bead molding. 3 More preferably, 15 kg / m 3 and preferably 100 kg / m 3 Less than or equal to 70 kg / m 3 More preferably, 50 kg / m or less 3 More preferably, 30 kg / m or less 3 Less than 20 kg / m, particularly preferably 20 kg / m 3 The following is the result. The bulk density of the expanded beads is calculated as follows: randomly take expanded beads from the expanded bead group and place them in a measuring cylinder with a volume of 1 L. A large number of expanded beads are placed in a natural pile up to the 1 L mark, and the mass W1 [g] of the expanded beads is divided by the volume V1 (1 [L]) (W1 / V1) to obtain the unit of [kg / m 3 The bulk density of the expanded beads can be calculated by converting the bulk density into the mass per unit area (mass mass). Conventionally, when the bulk density of the expanded beads is low, the mold releasability is easily impaired. 3 Even in the case of expanded beads having a low bulk density such as those below, the releasability is good.

[0042] The expanded beads preferably have a crystal structure in which a melting peak due to melting inherent to the polypropylene resin (i.e., resin-specific peak) and one or more melting peaks (i.e., high-temperature peaks) appear on the higher temperature side of the melting peak in the DSC curve obtained when the expanded beads are heated from 23°C to 200°C at a heating rate of 10°C / min. The DSC curve is obtained by differential scanning calorimetry (DSC) in accordance with JIS K7121:1987 using 1 to 3 mg of expanded beads as a test sample. The resin-specific peak is a melting peak due to melting inherent to the polypropylene resin constituting the foamed core layer, and is considered to be due to endothermic heat at the time of melting of the crystals inherent to the polypropylene resin. On the other hand, the melting peak on the higher temperature side of the resin-specific peak (i.e., high-temperature peak) is a melting peak that appears on the higher temperature side than the resin-specific peak in the DSC curve. When this high-temperature peak appears, it is presumed that secondary crystals exist in the resin. In addition, in the DSC curve obtained by heating the expanded beads from 23°C to 200°C at a heating rate of 10°C / min (i.e., the first heating), cooling from 200°C to 23°C at a cooling rate of 10°C / min, and then heating again from 23°C to 200°C at a heating rate of 10°C / min (i.e., the second heating), only the melting peak due to melting specific to the polypropylene resin constituting the expanded beads is observed, so that the resin-specific peak and the high-temperature peak can be distinguished. The apex temperature of this resin-specific peak may differ slightly between the first and second heatings, but the difference is usually within 5°C.

[0043] <High-temperature peak heat of fusion ΔH2> The heat of fusion ΔH2 of the high-temperature peak of the polypropylene resin expanded beads is, from the viewpoint of the mechanical strength and moldability of the expanded beads, preferably 5 J / g or more, more preferably 8 J / g or more, even more preferably 10 J / g or more, and is preferably 40 J / g or less, more preferably 30 J / g or less, even more preferably 20 J / g or less, and even more preferably 18 J / g or less.

[0044] The method for measuring the heat of fusion ΔH2 of the high-temperature peak of polypropylene resin expanded beads is explained using the DSC curve illustrated in Figure 2. Point α at 80°C on the DSC curve is connected to point β at the melting end temperature T of the expanded beads to obtain a straight line L1. Next, a straight line L2 parallel to the vertical axis of the graph is drawn from point γ on the DSC curve, which corresponds to the valley between the resin-specific peak P1 and the high-temperature peak P2, and the point where the straight lines L1 and L2 intersect is defined as δ. Point γ can also be considered as the maximum point existing between the resin-specific peak P1 and the high-temperature peak P2. The area (2) of the high-temperature peak P2 of the expanded beads is the heat of fusion ΔH2 of the high-temperature peak P2, and is calculated as the area surrounded by the DSC curve showing the high-temperature peak P2, the line segment (δ-β), and the line segment (γ-δ). The area (1) of the resin-specific peak P1 of the expanded beads is the heat of fusion ΔH1 of the resin-specific peak P1, and is calculated as the area surrounded by the DSC curve showing the resin-specific peak P1, the line segment (α-δ), and the line segment (γ-δ).

[0045] [Method of manufacturing polypropylene resin foam particles] The method for producing the expanded polypropylene resin particles is not particularly limited, but the particles can be produced, for example, by a method including the following steps (A) to (D). Step (A): A granulation step in which a polypropylene-based resin (C) constituting a core layer and a polypropylene-based resin (S) constituting a coating layer are melt-kneaded and co-extruded into a strand shape, and then cut to obtain multilayer resin particles having a non-foamed core layer and a coating layer coating the core layer; Step (B): A dispersion step of dispersing the multilayer resin particles in a dispersion medium in a sealed container; Step (C): A foaming agent impregnation step of impregnating the multilayer resin particles with a foaming agent; and Step (D): An expansion step in which the expandable multilayer resin particles impregnated with a blowing agent are released from the sealed container together with a dispersion medium into an atmosphere having a lower pressure than the pressure inside the sealed container, thereby expanding at least the core layer into a foamed core layer, thereby producing foamed particles.

[0046] <Process (A)> In step (A), for example, an extruder having a core layer forming extruder, a coating layer forming extruder, and a die for forming a multilayer strand installed on the outlet side of these extruders can be used. The extruder for forming the core layer is supplied with the polypropylene resin (C) constituting the core layer and additives added as necessary, and melt-kneaded to form a core layer forming melt-kneaded material, and the extruder for forming the coating layer is supplied with the polypropylene resin (S) constituting the coating layer and additives added as necessary, and melt-kneaded to form a coating layer forming melt-kneaded material. The core layer forming melt-kneaded material and the coating layer forming melt-kneaded material are introduced into the die for forming the multilayer strand and merged to form a composite having a core-sheath structure, which is composed of a non-foamed core layer and a non-foamed coating layer covering the core layer. The composite is extruded in the form of a strand from a small hole in a die attached to the tip of the extruder, cooled in water, and then cut to a predetermined mass with a pelletizer (strand cut method), thereby obtaining multilayer resin particles having a non-foamed core layer and a coating layer that coats the core layer. In addition to the above-mentioned method, the extruded composite can also be cut by an underwater cut method in which the composite is extruded into water and cut, or a hot cut method in which the composite is extruded into air and cut immediately afterwards.

[0047] (Particle size of multi-layer resin particles) The particle size of the multilayered resin particles is preferably 0.1 to 3.0 mm, and more preferably 0.3 to 1.5 mm.

[0048] (mass of multi-layer resin particles) The average mass of the multilayered resin particles is preferably adjusted to be 0.1 to 20 mg, more preferably 0.2 to 10 mg, even more preferably 0.3 to 5 mg, and even more preferably 0.4 to 2 mg.

[0049] (mass ratio of coating layer to core layer) When producing multilayer resin particles, the mass ratio of the coating layer to the non-expanded core layer (coating layer / core layer) is preferably 0.5 / 99.5 to 20 / 80, more preferably 1 / 99 to 15 / 85, and even more preferably 3 / 97 to 10 / 90, from the viewpoint of the fusibility of the expanded beads.

[0050] (Additives) Additives such as bubble regulators, flame retardants, flame retardant auxiliaries, bubble nucleating agents, plasticizers, antistatic agents, antioxidants, UV inhibitors, light stabilizers, conductive fillers, and antibacterial agents can be added to the multilayer resin particles having a non-foamed core layer and a coating layer, as necessary. When additives are added, they can be added in step (A). Examples of bubble regulators include inorganic powders such as talc, mica, zinc borate, calcium carbonate, silica, titanium oxide, gypsum, zeolite, borax, aluminum hydroxide, and carbon; and organic powders such as phosphoric acid-based nucleating agents, phenol-based nucleating agents, amine-based nucleating agents, and polyethylene fluoride-based resin powder. When a bubble regulator is added, the content of the bubble regulator in the multilayer resin particles is preferably 0.01 to 1 part by mass relative to 100 parts by mass of the multilayer resin particles.

[0051] <Process (B)> In the step (B), the multilayered resin particles can be dispersed in a dispersion medium using, for example, a stirrer in a container that is sealable and can withstand heat and pressure, such as an autoclave.

[0052] The dispersion medium is not particularly limited as long as it does not dissolve the multilayer resin particles. Examples of the dispersion medium include water, ethylene glycol, glycerin, and alcohols such as methanol and ethanol, and among these, water is preferred.

[0053] In step (B), in order to prevent the multilayer resin particles from fusing together, it is preferable to further add a dispersant to the dispersion medium. Examples of the dispersant include organic dispersants such as polyvinyl alcohol, polyvinylpyrrolidone, and methyl cellulose; and poorly soluble inorganic salts such as aluminum oxide, zinc oxide, kaolin, mica, magnesium phosphate, and tricalcium phosphate. These can be used alone or in combination of two or more. Among these, from the viewpoint of ease of handling, poorly soluble inorganic salts are preferable, and kaolin is more preferable. When a dispersant is added, it is preferable to add about 0.001 to 5 parts by mass of the dispersant relative to 100 parts by mass of the multilayer resin particles.

[0054] A surfactant may be further added to the dispersion medium. Examples of the surfactant include sodium dodecylbenzenesulfonate, sodium alkylsulfonate, sodium oleate, sodium lauryl sulfate, sodium polyoxyethylene alkyl ether phosphate, sodium polyoxyethylene alkyl ether sulfate, and other anionic surfactants and nonionic surfactants that are generally used in suspension polymerization. When a surfactant is added, it is preferable to add the surfactant in an amount of about 0.001 to 1 part by mass relative to 100 parts by mass of the multilayer resin particles.

[0055] <Process (C)> In step (C), for example, the polypropylene resin (C) constituting the core layer is heated to a temperature equal to or higher than the softening temperature, and impregnated with a foaming agent to obtain expandable multilayer resin particles.

[0056] The foaming agent is not particularly limited as long as it can foam the multilayer resin particles. Examples of the foaming agent include inorganic physical foaming agents such as air, nitrogen, carbon dioxide, argon, helium, oxygen, and neon, aliphatic hydrocarbons such as propane, normal butane, isobutane, normal pentane, isopentane, and normal hexane, alicyclic hydrocarbons such as cyclohexane and cyclopentane, halogenated hydrocarbons such as chlorofluoromethane, trifluoromethane, 1,1-difluoroethane, 1,1,1,2-tetrafluoroethane, methyl chloride, ethyl chloride, and methylene chloride, and organic physical foaming agents such as dialkyl ethers such as dimethyl ether, diethyl ether, and methyl ethyl ether. Among these, inorganic physical foaming agents that do not destroy the ozone layer and are inexpensive are preferred, and nitrogen, air, and carbon dioxide are more preferred, and carbon dioxide is particularly preferred. These are used alone or in combination of two or more kinds.

[0057] The amount of foaming agent added is determined taking into consideration the desired bulk density of the expanded particles, the type of polypropylene resin, the type of foaming agent, etc., but typically, for organic physical foaming agents, the amount is preferably 5 to 50 parts by mass, and for inorganic physical foaming agents, the amount is preferably 0.1 to 30 parts by mass, and more preferably 0.5 to 15 parts by mass, per 100 parts by mass of the multilayer resin particles.

[0058] The heating temperature in step (C) is preferably equal to or higher than the melting point of the polypropylene resin (C) and equal to or lower than the melting point + 80° C., specifically, preferably 100° C. to 230° C. The time for which the resin is held at the heating temperature is preferably 1 minute or more, more preferably 20 minutes or more, and preferably 100 minutes or less, more preferably 60 minutes or less.

[0059] <Process (D)> In step (D), for example, the expandable multilayer resin particles that have been impregnated with a blowing agent in step (C) and heated are released from the sealed container into an atmosphere having a lower pressure than the pressure inside the sealed container, and at least the core layer is expanded to form a foamed core layer, thereby producing expanded beads. Specifically, while maintaining the pressure in the sealed container at or above the vapor pressure of the blowing agent, one end of the sealed container below the water surface is opened, and the expandable multilayer resin particles impregnated with the blowing agent are released from the sealed container together with the dispersion medium into an atmosphere of a lower pressure than the pressure in the sealed container, usually atmospheric pressure, and at least the core layer of the expandable multilayer resin particles is expanded to form a foamed core layer, thereby producing expanded beads of a multilayer structure having a foamed core layer and a coating layer that covers the foamed core layer.Furthermore, the expandable multilayer resin particles that have been subjected to step (C) can also be produced by cooling and removing them, and then heating the expandable multilayer resin particles with a heating medium such as hot air or steam to expand them.

[0060] In the step (D), the temperature during foaming is usually preferably 110° C. to 170° C. The pressure inside the sealed container is preferably 0.5 MPa (G) or more and 5 MPa (G) or less.

[0061] The above steps (B) to (D) are preferably carried out as a series of steps in a single sealed container. However, they can also be separate steps in which the multilayer resin particles, etc. are removed for each step and then put back into the sealed container to carry out the next step.

[0062] In addition, expanded beads having a crystalline structure in which a resin-specific melting peak (resin-specific peak) and one or more melting peaks (high-temperature peaks) appear on the high-temperature side of the resin in the first DSC curve obtained by differential scanning calorimetry (DSC) of the expanded beads can be obtained, for example, as follows. During heating in the expanded beads production process, a first-stage holding step is performed in which the temperature is held at a temperature of (melting point of polypropylene resin (C) -20°C) or more and less than (melting end temperature of polypropylene resin (C)) for a sufficient time, preferably about 10 to 60 minutes. Thereafter, the temperature is adjusted to (melting point of polypropylene resin (C) -15°C) to (melting end temperature of polypropylene resin (C) +10°C). If necessary, a second-stage holding step is performed in which the temperature is held for a further sufficient time, preferably about 10 to 60 minutes. Next, the expandable resin particles containing a foaming agent are released from the sealed container under low pressure and expanded to obtain expanded beads having the above-mentioned crystal structure. The expansion is preferably performed at a temperature of (melting point of polypropylene resin (C) -10°C) or more, more preferably at (melting point of polypropylene resin (C)) or more and less than (melting point of polypropylene resin (C) +20°C).

[0063] The expanded beads obtained as described above can be pressurized with air to increase the internal pressure, and then heated with steam or the like to expand (two-stage expansion) to give expanded beads with a higher expansion ratio (lower apparent density).

[0064] [Polypropylene resin foamed bead molding] The polypropylene-based resin expanded bead molding obtained by using the polypropylene-based resin expanded beads of the present invention (hereinafter, also simply referred to as a polypropylene-based resin expanded bead molding or an expanded bead molding) can be obtained by molding the polypropylene-based resin expanded beads of the present invention in a mold.

[0065] The in-mold molding method can be carried out by filling the foamed beads in a mold and molding them by heating using a heating medium such as steam. Specifically, after filling the foamed beads in a mold, a heating medium such as steam is introduced into the mold to heat and expand the foamed beads and fuse them together to obtain a foamed bead molding having the shape of the molding space. In addition, the in-mold molding in the present invention is preferably molded by a pressure molding method (for example, JP-B-51-22951) in which the foamed beads are pressurized in advance with a pressurized gas such as air to increase the pressure in the bubbles of the foamed beads and adjust the pressure in the foamed beads to a pressure 0.01 to 0.3 MPa higher than atmospheric pressure, and then the foamed beads are filled into the mold under atmospheric pressure or reduced pressure, and then a heating medium such as steam is supplied into the mold to heat and fuse the foamed beads. Molding can also be performed by a compression filling molding method (JP-B-4-46217) in which a mold pressurized to atmospheric pressure or higher by a compressed gas is filled with expanded beads, which have been pressurized to the atmospheric pressure or higher, and then a heating medium such as steam is supplied into the cavity to heat and fuse the expanded beads. Molding can also be performed by a normal pressure filling molding method (JP-B-6-49795) in which expanded beads with high secondary expansion power obtained under special conditions are filled into the cavity of a mold under atmospheric pressure or reduced pressure, and then a heating medium such as steam is supplied to heat and fuse the expanded beads, or a combination of the above methods (JP-B-6-22919).

[0066] <density> The density of the polypropylene resin foamed bead molding is preferably 10 kg / m from the viewpoint of achieving a good balance between mechanical strength and light weight. 3 More preferably, 15 kg / m 3 More preferably, 20 kg / m 3 and preferably 150 kg / m 3 Less than or equal to 100 kg / m 3 More preferably, 50 kg / m or less 3 Less than 35 kg / m, particularly preferably 35 kg / m 3 The following is the result. The density of the expanded bead molding is calculated by dividing the mass (g) of the expanded bead molding by the volume (L) calculated from the outer dimensions of the molding, and converting the result into units. If it is not easy to calculate the volume from the outer dimensions of the molding, the volume of the molding can be calculated by a submersion method.

[0067] <50% compressive stress> From the viewpoint of rigidity, the 50% compressive stress of the polypropylene resin foamed bead molding is preferably 50 kPa or more, more preferably 100 kPa or more, even more preferably 150 kPa or more, and is preferably 500 kPa or less, more preferably 400 kPa or less, even more preferably 300 kPa or less. The 50% compressive stress of the expanded bead molding is measured in accordance with JIS K6767:1999. EXAMPLES

[0068] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0069] The following measurements and evaluations were carried out for the resins, expanded beads, and expanded bead moldings used in the examples and comparative examples. The physical properties of the expanded beads were measured using expanded beads that had been conditioned by standing for 24 hours at a relative humidity of 50%, 23°C, and 1 atm. The physical properties of the expanded bead moldings were measured and evaluated using expanded beads that had been conditioned by standing for 12 hours at a relative humidity of 50%, 80°C, and 1 atm after demolding.

[0070] [Measurement method] <Polypropylene resin> (monomer component content) The content of the monomer component of the polypropylene resin was determined by a known method of determining it by IR spectrum. Specifically, it was determined by the method described in Polymer Analysis Handbook (edited by Polymer Analysis Research Roundtable of the Japan Analytical Chemistry Society, published in January 1995, published by Kinokuniya Shoten, page numbers and item names: 615-616 "II.2.3 2.3.4 Propylene / ethylene copolymer", 618-619 "II.2.3 2.3.5 Propylene / butene copolymer"), that is, by a method of quantitatively determining the relationship between the value of the absorbance of ethylene and butene corrected by a predetermined coefficient and the thickness of a film-like test piece. More specifically, first, the polypropylene resin was hot pressed in a 180°C environment to form a film-like shape, and a plurality of test pieces with different thicknesses were prepared. Next, the IR spectrum of each test piece was measured to determine the 722 cm ray originating from ethylene. -1 and 733 cm -1 Absorbance at (A 722 , A 733 ) and 766 cm -1 Absorbance at (A 766 ) was read. Next, for each test piece, the ethylene component content in the polypropylene-based resin was calculated using the following formulas (1) to (3). The arithmetic average of the ethylene component contents obtained for each test piece was taken as the ethylene component content (unit: mass%) in the polypropylene-based resin. (K 733 )c=1 / 0.96{(K´ 733 )a-0.268(K´ 722 )a}···(1) (K 722 )c=1 / 0.96{(K´ 722 )a-0.268(K´ 722 )a}···(2) Ethylene content (%) = 0.575 {(K' 722 )c+(K´ 733 )c}···(3) In the formulas (1) to (3), K´a is the apparent absorption coefficient at each wave number (K´a=A / ρt), K´c is the corrected absorption coefficient, A is the absorbance, and ρ is the density of the resin (unit: g / cm 3), t: thickness of the film-like test piece (unit: cm). The butene content in the polypropylene resin was calculated for each test piece using the following formula (4). The arithmetic mean of the butene contents obtained for each test piece was taken as the butene content (%) in the polypropylene resin. Butene content (%) = 12.3 (A 766 / L) · · · (4) In the formula (4), A is the absorbance, and L is the thickness (mm) of the film-like test piece.

[0071] (DSC characteristics of polypropylene resin (S)) In the second DSC curve of the polypropylene resin (S), the peak shape, the apex temperature Tmsl of the melting peak on the low temperature side, and the apex temperature Tmsh of the melting peak on the high temperature side of the polypropylene resin (S) were determined as follows. Based on JIS K7121:1987, the condition of the test piece was adjusted as follows: "(2) The case where the melting temperature is measured after a certain heat treatment" was adopted, and the condition-adjusted test piece was heated from 30°C to 200°C at a heating rate of 10°C / min to obtain a DSC curve. Specifically, about 3 mg of resin conditioned at 23°C and 50% RH for 24 hours or more was used as a test piece, and the test piece was heated from 30°C to 200°C at a heating rate of 10°C / min using a heat flux differential scanning calorimeter (Shimadzu Corporation, model number: DSC-60A), then cooled from 200°C to 30°C at a cooling rate of 10°C / min, and heated again from 30°C to 200°C at a heating rate of 10°C / min to obtain a DSC curve and observe the shape of the melting peak. Here, the double peak shape means that two melting peaks having different peak apex temperatures (melting points) appear, and the heat of fusion of both of the two melting peaks is 10 J / g or more. The heat of fusion is a value obtained by the method described below. For PP2, since only one melting peak appeared, that is, a single peak shape, the apex temperature of the melting peak was displayed only in the column of the apex temperature of the melting peak on the lower temperature side. The second DSC curve of PP1 is shown in Figure 1. The second DSC curve of PP2 is shown in Figure 3.

[0072] (Heat of fusion of polypropylene resin (S) ΔH) The heat of fusion of the polypropylene resin (S) was calculated as follows. The point on the DSC curve at a temperature of 80° C. was taken as α, and the point on the DSC curve corresponding to the melting end temperature was taken as β (see FIG. 1). l and the melting peak P h From point γ on the DSC curve, which is in the valley between points α and β, a straight line parallel to the vertical axis of the graph is drawn, and the point where it intersects with the straight line (α-β) connecting points α and β is defined as δ. l The area (A) of the melting peak P l The heat of fusion ΔHl of the low-temperature melting peak P l The melting peak P was calculated as the area enclosed by the DSC curve showing the melting peak P h The area (B) of the melting peak P h The heat of fusion of ΔHh is the melting peak P h The area was calculated as the area enclosed by the DSC curve showing the above, the line segment (δ-β) and the line segment (γ-δ). The total heat of fusion ΔHt is the melting peak P l The area (A) of the melting peak on the high temperature side P h In FIG. 1, it was calculated as the area surrounded by the straight line (α-β) and the DSC curve in the section between point α and point β. As for PP2, since it had a single-peak shape in which only one melting peak appeared, the heat of fusion of that melting peak was shown only in the column for the heat of fusion of the melting peak on the low temperature side.

[0073] (Melting point of polypropylene resin (C)) The melting point Tmc of the polypropylene resin (C) was determined based on JIS K7121:1987 by adopting "(2) the case where the melting temperature is measured after performing a certain heat treatment" as the conditioning of the test piece. Specifically, about 3 mg of the resin conditioned at 23°C and 50% RH for 24 hours or more was used as the test piece, and the test piece was heated from 30°C to 200°C at a heating rate of 10°C / min using a heat flux differential scanning calorimeter (Shimadzu Corporation, model number: DSC-60A), then cooled from 200°C to 30°C at a cooling rate of 10°C / min, and heated again from 30°C to 200°C at a heating rate of 10°C / min to obtain a DSC curve, and the apex temperature of the melting peak accompanying the melting of the resin on the DSC curve was obtained.

[0074] (MFR) The MFR of polypropylene resins was measured in accordance with JIS K7210-1:2014 at a temperature of 230°C and a load of 2.16 kg.

[0075] (Flexural modulus) The flexural modulus of polypropylene resin was measured according to JIS K7171:2016 by preparing a 4 mm thick sheet by heat pressing at 230°C, and cutting out a standard test piece of 80 mm length x 10 mm width x 4 mm thickness from the sheet. The radius R1 of the indenter and the radius R2 of the support were both 5 mm, the distance between the supports was 64 mm, and the test speed was 2 mm / min.

[0076] <Expanded particles> (Bulk density) The bulk density of the expanded beads was determined as follows: Expanded beads were randomly taken from the group of expanded beads after conditioning and placed in a measuring cylinder with a volume of 1 L, and a large number of expanded beads were placed up to the 1 L mark so as to form a natural pile-up state. The mass W1 [g] of the expanded beads was divided by the volume V1 (1 [L]) (W1 / V1) to obtain the unit of [kg / m 3 The bulk density of the expanded beads was calculated by converting the mass per unit area (mass per unit area) into the mass per unit area (mass per unit area).

[0077] (High temperature peak heat quantity) One expanded bead was taken from the group of expanded beads after the condition adjustment. This expanded bead was used as a test piece, and a DSC curve was obtained by heating the test piece from 23°C to 200°C at a heating rate of 10°C / min using a differential scanning calorimeter (specifically, manufactured by Shimadzu Corporation, model number: DSC-60A). An example of a DSC curve is shown in FIG. 2. The DSC curve shown in FIG. 2 has a resin-specific peak P1 and a high-temperature peak P2 on the high-temperature side thereof, and a line L1 was obtained by connecting point α at a temperature of 80°C on the DSC curve and point β at the melting end temperature T of the expanded beads. Next, a line L2 parallel to the vertical axis of the graph was drawn from point γ on the DSC curve corresponding to the valley between the resin-specific peak P1 and the high-temperature peak P2, and the point where the lines L1 and L2 intersect was defined as δ. The area (2) of the high-temperature peak P2 of the expanded beads is the heat of fusion ΔH2 of the high-temperature peak P2, and was calculated as the area of ​​the part surrounded by the DSC curve showing the high-temperature peak P2, the line segment (δ-β), and the line segment (γ-δ). The above measurement was performed for five expanded beads, and the arithmetic average values ​​are shown in Tables 3 and 4.

[0078] <Expanded bead molding> (Molded object density) The density of the expanded bead molding was determined by randomly cutting three rectangular specimens from the expanded bead molding, excluding the skin layer formed during molding, measuring the mass and volume of each specimen, and calculating the density of the three specimens by dividing the mass by the volume to convert the unit, and then calculating the arithmetic average value.

[0079] (50% compressive stress) The 50% compressive stress of the expanded bead molding was determined by cutting a rectangular specimen 50 mm long x 50 mm wide x 25 mm thick from the expanded bead molding, excluding the skin layer, and compressing this specimen at a rate of 10 mm / min according to JIS K6767:1999 using an RTF-1350 manufactured by A&D Co., Ltd. to determine the load at 50% strain, and dividing this by the pressure-receiving area of ​​the specimen to obtain the 50% compressive stress [kPa].

[0080] [Evaluation method] <Range of molding pressure that can be molded (moldability)> The moldability of the expanded beads was evaluated as follows. In the <Preparation of Expanded Bead Molded Body> described later, the molding pressure (steam pressure) was changed in increments of 0.01 MPa (G) to prepare expanded bead molded bodies, and the molding pressure at which the fusion property, appearance, and recovery property of the expanded bead molded body were all rated as "A" was determined as the range of molding pressures at which the moldable molding pressures could be obtained. The molding pressure scores (the number of molding pressures at which all evaluations were "A") are also shown in Tables 3 and 4. Since the molding temperature is adjusted by the molding pressure, the wider the range from the lower limit to the upper limit of the moldable molding pressure, the wider the range of the molding heating temperature at which the moldable molding pressure can be obtained. (Fusing ability) The fusion property of the expanded bead molding was evaluated by the following method. The expanded bead molding was folded and broken, the number of expanded beads present on the fracture surface (C1) and the number of broken expanded beads (C2) were determined, and the ratio of broken expanded beads to the above-mentioned expanded beads (C2 / C1 x 100) was calculated as the material destruction rate. Measurements were performed five times using different test pieces, and the material destruction rates for each were calculated and the arithmetic average was used to evaluate the fusion property according to the following criteria. A: Material destruction rate 90% or more B: Material destruction rate is 20% or more but less than 90% C: Material destruction rate less than 20% (Appearance (degree of gap)) A 100 mm x 100 mm rectangle was drawn in the center of the expanded bead molding, and a line was drawn diagonally from the corner of the rectangular area. The number of voids (gaps) measuring 1 mm x 1 mm or more on that line was counted, and the surface appearance of the expanded bead molding was evaluated according to the following criteria. A: The number of voids is less than 3 B: The number of voids is 3 or more but less than 5 C: 5 or more voids (Recoverability) The thickness of the central portion and the four corner portions of the obtained foamed bead molding was measured, and the ratio of the thickness of the central portion to the thickest portion among the four corner portions was calculated, and the recovery property was evaluated according to the following criteria. A: When the thickness ratio is 95% or more B: Thickness ratio is 90% or more and less than 95% C: Thickness ratio is less than 90%

[0081] <Range of molding pressure for good demolding (mold releasability)> In the evaluation of the above <Range of molding pressure that allows molding (moldability)>, within the range of molding pressure that allows molding, the mold releasability was evaluated according to the following criteria, and molding pressures that were evaluated as "A" were deemed to have good mold releasability. The molding pressure scores (the number of molding pressures that were evaluated as "A") are also shown in the table. The wider the range from the lower limit to the upper limit of the molding pressure that allows good mold releasability, the wider the range of molding heating temperatures that allows good mold releasability. A: Demolding is possible without using demolding assist such as demolding pins or demolding air. B: Demolding is not possible without using demolding assist such as demolding pins or demolding air.

[0082] [Raw materials] The polypropylene-based resins (S) and (C) used in the examples and comparative examples are shown in Tables 1 and 2. In the tables, PP1 is a propylene-based copolymer containing ethylene, propylene, and butene, and is manufactured by TPC Corporation under the grade name FL7540L. In the table, PP5 is a mixed resin (mass ratio is 50% by mass:50% by mass, with the total of both being 100% by mass) of an ethylene-propylene random copolymer (ethylene 3.1% by mass, melting point 142°C) and an ethylene-propylene random copolymer (ethylene 1.4% by mass, melting point 153°C). PP6 is a mixed resin (mass ratio is 50% by mass:50% by mass, with the total of both being 100% by mass) of an ethylene-propylene random copolymer (ethylene 4.2% by mass, melting point 134°C) and a linear low-density polyethylene (density 0.91 g / cm 3 110° C.) (the mass ratio is ethylene-propylene random copolymer:linear low-density polyethylene=67% by mass:33% by mass, with the total of both being 100% by mass).

[0083] [Table 1]

[0084] [Table 2]

[0085] Examples 1 to 5 and Comparative Examples 1 to 9 <Preparation of foam particles> Using the polypropylene resin (C) of the type shown in Tables 3 and 4, the resin melt-kneaded material for forming the core layer was melt-kneaded in the extruder for forming the core layer at a maximum set temperature of 245°C to obtain a resin melt-kneaded material for forming the core layer. Also, the polypropylene resin (S) of the type shown in Tables 3 and 4 was melt-kneaded in the extruder for forming the coating layer at a maximum set temperature of 245°C to obtain a resin melt-kneaded material for forming the coating layer. Next, each resin melt-kneaded material was extruded from the tip of the co-extrusion die from the extruder for forming the core layer and the extruder for forming the coating layer. At this time, each resin melt-kneaded material was merged in the die to form a sheath-core type composite consisting of a non-foamed core layer and a non-foamed coating layer that covers the outer surface of the core layer. The composite was extruded in a strand shape from the hole of the die attached to the tip of the extruder, and the strand-shaped material was cooled with water while being taken up, and then cut into a mass of about 1.0 mg with a pelletizer. In this way, a multilayer resin particle consisting of a core layer and a coating layer that covers the core layer was obtained. The mass ratio of the core layer to the coating layer in the multi-layered resin particles was 95:5 (i.e., the mass ratio of the coating layer was 5%). In the production of the multi-layered resin particles, zinc borate was supplied to the extruder for forming the core layer as a bubble adjuster, and 500 ppm by mass of zinc borate was contained in the polypropylene resin.

[0086] (One-stage foaming process) 1 kg of multilayer resin particles was charged into a 5L sealed container together with 3L of water as a dispersion medium, and 0.3 parts by mass of kaolin as a dispersant, 0.0001 parts by mass of aluminum sulfate as a dispersion aid, and 0.004 parts by mass of a surfactant (sodium alkylbenzene sulfonate) were added to the sealed container relative to 100 parts by mass of the multilayer resin particles. After carbon dioxide was added to the sealed container as a foaming agent, the sealed container was sealed, and the contents of the sealed container were heated to the foaming temperature shown in Tables 3 and 4 while stirring. The pressure inside the container at this time (i.e., impregnation pressure, carbon dioxide pressure) was as shown in Tables 3 and 4. After maintaining the temperature at the same temperature for 15 minutes, the contents of the container were released under atmospheric pressure to obtain expanded particles. The expanded particles were dried at 23°C for 24 hours. In this way, expanded particles having the bulk ratios shown in Tables 3 and 4 were obtained.

[0087] (Two-stage foaming process) Next, the expanded beads were placed in a pressure vessel (specifically, a metal drum), and air was forced into the pressure vessel to increase the pressure inside the vessel, thereby impregnating the air bubbles and increasing the internal pressure inside the bubbles of the expanded beads. Next, steam was supplied to the expanded beads (first-stage expanded beads) taken out of the pressure vessel so that the pressure inside the pressure vessel (i.e., drum pressure) became the pressures shown in Tables 3 and 4, and the expanded beads were heated under atmospheric pressure. The pressure inside the bubbles (i.e., internal pressure) of the first-stage expanded beads taken out of the pressure vessel had the values ​​shown in Tables 3 and 4. As a result, the apparent density of the first-stage expanded beads was reduced, and expanded beads (second-stage expanded beads) having the bulk ratios shown in Tables 3 and 4 were obtained.

[0088] <Preparation of foamed bead molded body> For the production of the foamed bead molded body, foamed beads dried at 23°C for 24 hours were used. First, an internal pressure of 0.25 MPa (G) was applied to the foamed beads by compressed air, and then the foamed beads were filled into a vertical molding machine having a cavity of 300 mm long x 250 mm wide x 60 mm thick, with the cracking amount adjusted to 20% (i.e., 12 mm), and the mold was clamped and steam was supplied from both sides of the mold for 5 seconds to perform an exhaust process of preheating. Note that the vertical molding machine is a molding machine in which the opening direction of the mold is vertical (height direction). Then, one-sided heating was performed by supplying steam from one side of the mold until a pressure 0.08 MPa (G) lower than the predetermined molding pressure was reached. Next, one-sided heating was performed by supplying steam from the other side of the mold until a pressure 0.04 MPa (G) lower than the molding pressure was reached, and then heating (i.e., main heating) was performed until the predetermined molding pressure was reached. After heating, the pressure was released and the molded article was cooled with water until the surface pressure due to the expanding force of the molded article reached 0.04 MPa (G), after which the mold was released from the die to obtain an expanded bead molded article.

[0089] [Table 3]

[0090] [Table 4]

[0091] As can be seen from Table 3, the expanded beads of the present invention have a wide range of molding pressures and good releasability over the entire range of molding pressures. When an expanded bead molded article sticks to the mold when it is removed from the mold, a releasing aid such as a releasing pin or releasing air is generally used, but the expanded beads of the present invention do not require a releasing aid, thereby improving production efficiency. As can be seen from Table 4, in Comparative Examples 1, 7, and 9, in which the polypropylene resin (S) had a single peak rather than a double peak and did not satisfy the above requirement (1), it was difficult to achieve both releasability and moldability. As can be seen from the comparison between Comparative Example 1 and Comparative Example 9, the releasability was particularly likely to be poor in the expanded beads having a low bulk density. The polypropylene resin (S) had a melting peak apex temperature on the higher temperature side of less than 138° C., and Comparative Examples 2, 3 and 8 did not satisfy the above requirement (3), and thus had poor releasability. Comparative Example 5, in which the difference [Tmc-Tmsh] between the melting point Tmc of the polypropylene-based resin (C) and the apex temperature Tmsh of the melting peak on the higher temperature side of the polypropylene-based resin (S) exceeded 15°C and did not satisfy the above requirement (4), also had poor releasability. Comparative Example 4, in which the apex temperature of the melting peak on the higher temperature side of the polypropylene-based resin (S) was less than 138°C and did not satisfy the above requirement (3), and the difference [Tmc-Tmsh] between the melting point Tmc of the polypropylene-based resin (C) and the apex temperature Tmsh of the melting peak on the higher temperature side of the polypropylene-based resin (S) exceeded 15°C and did not satisfy the above requirement (4), had significantly poor releasability. [Industrial Applicability]

[0092] The expanded beads of the present invention are useful because they can be molded under a wide range of molding pressures and have excellent releasability, and therefore can be used with general-purpose equipment instead of special equipment, since they exhibit excellent releasability.

Claims

1. A polypropylene-based resin expanded particle having an expanded core layer composed of a polypropylene-based resin (C) and a coating layer composed of a polypropylene-based resin (S), The melting point Tmc of the polypropylene resin (C) is 125°C or higher and 170°C or lower, (1) The polypropylene resin (S) has a melting peak on the low temperature side and a melting peak on the high temperature side in a second DSC curve obtained by heat flux differential scanning calorimetry, in which the polypropylene resin (S) is heated from 30° C. to 200° C. at a heating rate of 10° C. / min, then cooled from 200° C. to 30° C. at a cooling rate of 10° C. / min, and heated again from 30° C. to 200° C. at a heating rate of 10° C. / min. (2) The apex temperature (Tmsl) of the melting peak on the low temperature side of the polypropylene-based resin (S) is lower than the melting point (Tmc) of the polypropylene-based resin (C); (3) The polypropylene resin (S) has a melting peak on the high temperature side (Tmsh) of 138° C. or higher; (4) The difference [Tmc-Tmsh] between the melting point Tmc of the polypropylene-based resin (C) and the apex temperature Tmsh of the melting peak on the higher temperature side of the polypropylene-based resin (S) is -10°C or more and 15°C or less, The polypropylene-based resin expanded particles have a difference [Tmc-Tmsl] between the melting point Tmc of the polypropylene-based resin (C) and the apex temperature Tmsl of the melting peak on the low temperature side of the polypropylene-based resin (S), which is greater than 0°C and not greater than 25°C.

2. A polypropylene-based resin foamed particle having a foamed core layer composed of a polypropylene-based resin (C) and a coating layer composed of a polypropylene-based resin (S), The melting point Tmc of the polypropylene resin (C) is 125°C or higher and 170°C or lower, (1) The polypropylene resin (S) has a melting peak on the low temperature side and a melting peak on the high temperature side in a second DSC curve obtained by heat flux differential scanning calorimetry, in which the polypropylene resin (S) is heated from 30° C. to 200° C. at a heating rate of 10° C. / min, then cooled from 200° C. to 30° C. at a cooling rate of 10° C. / min, and heated again from 30° C. to 200° C. at a heating rate of 10° C. / min. (2) The apex temperature (Tmsl) of the melting peak on the low temperature side of the polypropylene-based resin (S) is lower than the melting point (Tmc) of the polypropylene-based resin (C); (3) The polypropylene resin (S) has a melting peak on the high temperature side (Tmsh) of 138° C. or higher; (4) The difference [Tmc-Tmsh] between the melting point Tmc of the polypropylene-based resin (C) and the apex temperature Tmsh of the melting peak on the higher temperature side of the polypropylene-based resin (S) is -10°C or more and 15°C or less, The polypropylene-based resin expanded particles, wherein the polypropylene-based resin (S) has a vertex temperature Tmsl of a melting peak on the low temperature side of 125°C or higher and 135°C or lower.

3. A polypropylene-based resin foamed particle having a foamed core layer composed of a polypropylene-based resin (C) and a coating layer composed of a polypropylene-based resin (S), The melting point Tmc of the polypropylene resin (C) is 125°C or higher and 170°C or lower, (1) The polypropylene resin (S) has a melting peak on the low temperature side and a melting peak on the high temperature side in a second DSC curve obtained by heat flux differential scanning calorimetry, in which the polypropylene resin (S) is heated from 30° C. to 200° C. at a heating rate of 10° C. / min, then cooled from 200° C. to 30° C. at a cooling rate of 10° C. / min, and heated again from 30° C. to 200° C. at a heating rate of 10° C. / min. (2) The apex temperature (Tmsl) of the melting peak on the low temperature side of the polypropylene-based resin (S) is lower than the melting point (Tmc) of the polypropylene-based resin (C); (3) The polypropylene resin (S) has a melting peak on the high temperature side (Tmsh) of 138° C. or higher; (4) The difference [Tmc-Tmsh] between the melting point Tmc of the polypropylene-based resin (C) and the apex temperature Tmsh of the melting peak on the higher temperature side of the polypropylene-based resin (S) is -10°C or more and 15°C or less, The polypropylene-based resin expanded particles have a ratio [ΔHh / ΔHt] of the heat of fusion (ΔHh) of a melting peak on the high temperature side of the polypropylene-based resin (S) to the total heat of fusion (ΔHt) of the polypropylene-based resin (S) of 0.35 or more and 0.80 or less.

4. A polypropylene-based resin foamed particle having a foamed core layer composed of a polypropylene-based resin (C) and a coating layer composed of a polypropylene-based resin (S), The melting point Tmc of the polypropylene resin (C) is 125°C or higher and 170°C or lower, (1) The polypropylene resin (S) has a melting peak on the low temperature side and a melting peak on the high temperature side in a second DSC curve obtained by heat flux differential scanning calorimetry, in which the polypropylene resin (S) is heated from 30° C. to 200° C. at a heating rate of 10° C. / min, then cooled from 200° C. to 30° C. at a cooling rate of 10° C. / min, and heated again from 30° C. to 200° C. at a heating rate of 10° C. / min. (2) The apex temperature (Tmsl) of the melting peak on the low temperature side of the polypropylene-based resin (S) is lower than the melting point (Tmc) of the polypropylene-based resin (C); (3) The polypropylene resin (S) has a melting peak on the high temperature side (Tmsh) of 138° C. or higher; (4) The difference [Tmc-Tmsh] between the melting point Tmc of the polypropylene-based resin (C) and the apex temperature Tmsh of the melting peak on the higher temperature side of the polypropylene-based resin (S) is -10°C or more and 15°C or less, The polypropylene-based resin expanded particles have a difference [Tmsh-Tmsl] between a top temperature Tmsh of a melting peak on a higher temperature side and a top temperature Tmsl of a melting peak on a lower temperature side of the polypropylene-based resin (S) of 10°C or more and 20°C or less.

5. The expanded polypropylene resin particles according to any one of claims 1 to 4, wherein the melting point Tmc of the polypropylene resin (C) is 130°C or higher and 150°C or lower.

6. The polypropylene-based resin expanded particles according to any one of claims 1 to 5, wherein the polypropylene-based resin (S) is a propylene-based copolymer containing a propylene component as a main component and an ethylene component and a butene component.

7. The bulk density of the polypropylene resin foam particles is 10 kg / m ３ More than 30kg / m ３ The expanded polypropylene resin particles according to any one of claims 1 to 6, wherein:

8. A method for producing expanded polypropylene resin particles, comprising the steps of: A step of granulating multilayer resin particles having a core layer made of a polypropylene-based resin (C) and a coating layer made of a polypropylene-based resin (S) that coats the core layer; dispersing the multilayered resin particles in a dispersion medium in a sealed container; impregnating the multilayered resin particles dispersed in the dispersion medium with a foaming agent; and a step of expanding the multilayer resin particles containing the foaming agent to produce expanded particles, The melting point Tmc of the polypropylene resin (C) is 125°C or higher and 170°C or lower, (1) The polypropylene resin (S) has a melting peak on the low temperature side and a melting peak on the high temperature side in a second DSC curve obtained by heat flux differential scanning calorimetry, in which the polypropylene resin (S) is heated from 30° C. to 200° C. at a heating rate of 10° C. / min, then cooled from 200° C. to 30° C. at a cooling rate of 10° C. / min, and heated again from 30° C. to 200° C. at a heating rate of 10° C. / min. (2) The apex temperature (Tmsl) of the melting peak on the low temperature side of the polypropylene-based resin (S) is lower than the melting point (Tmc) of the polypropylene-based resin (C); (3) The polypropylene resin (S) has a melting peak on the high temperature side (Tmsh) of 138° C. or higher; (4) The difference [Tmc-Tmsh] between the melting point Tmc of the polypropylene-based resin (C) and the apex temperature Tmsh of the melting peak on the higher temperature side of the polypropylene-based resin (S) is -10°C or more and 15°C or less, The method for producing expanded polypropylene-based resin beads, wherein the difference [Tmc - Tmsl] between the melting point Tmc of the polypropylene-based resin (C) and the apex temperature Tmsl of the melting peak on the low temperature side of the polypropylene-based resin (S) is more than 0°C and 25°C or less.

9. A method for producing expanded polypropylene-based resin particles, comprising the steps of: A step of granulating multilayer resin particles having a core layer made of a polypropylene-based resin (C) and a coating layer made of a polypropylene-based resin (S) that coats the core layer; dispersing the multilayered resin particles in a dispersion medium in a sealed container; impregnating the multilayered resin particles dispersed in the dispersion medium with a foaming agent; and a step of expanding the multilayer resin particles containing the foaming agent to produce expanded particles, The melting point Tmc of the polypropylene resin (C) is 125°C or higher and 170°C or lower, (1) The polypropylene resin (S) has a melting peak on the low temperature side and a melting peak on the high temperature side in a second DSC curve obtained by heat flux differential scanning calorimetry, in which the polypropylene resin (S) is heated from 30° C. to 200° C. at a heating rate of 10° C. / min, then cooled from 200° C. to 30° C. at a cooling rate of 10° C. / min, and heated again from 30° C. to 200° C. at a heating rate of 10° C. / min. (2) The apex temperature (Tmsl) of the melting peak on the low temperature side of the polypropylene-based resin (S) is lower than the melting point (Tmc) of the polypropylene-based resin (C); (3) The polypropylene resin (S) has a melting peak on the high temperature side (Tmsh) of 138° C. or higher; (4) The difference [Tmc-Tmsh] between the melting point Tmc of the polypropylene-based resin (C) and the apex temperature Tmsh of the melting peak on the higher temperature side of the polypropylene-based resin (S) is -10°C or more and 15°C or less, The polypropylene resin (S) has a melting peak apex temperature (Tmsl) of 125°C or higher and 135°C or lower.

10. A method for producing expanded polypropylene-based resin particles, comprising the steps of: A step of granulating multilayer resin particles having a core layer made of a polypropylene-based resin (C) and a coating layer made of a polypropylene-based resin (S) that coats the core layer; dispersing the multilayered resin particles in a dispersion medium in a sealed container; impregnating the multilayered resin particles dispersed in the dispersion medium with a foaming agent; and a step of expanding the multilayer resin particles containing the foaming agent to produce expanded particles, The melting point Tmc of the polypropylene resin (C) is 125°C or higher and 170°C or lower, (1) The polypropylene resin (S) has a melting peak on the low temperature side and a melting peak on the high temperature side in a second DSC curve obtained by heat flux differential scanning calorimetry, in which the polypropylene resin (S) is heated from 30° C. to 200° C. at a heating rate of 10° C. / min, then cooled from 200° C. to 30° C. at a cooling rate of 10° C. / min, and heated again from 30° C. to 200° C. at a heating rate of 10° C. / min. (2) The apex temperature (Tmsl) of the melting peak on the low temperature side of the polypropylene-based resin (S) is lower than the melting point (Tmc) of the polypropylene-based resin (C); (3) The polypropylene resin (S) has a melting peak on the high temperature side (Tmsh) of 138° C. or higher; (4) The difference [Tmc-Tmsh] between the melting point Tmc of the polypropylene-based resin (C) and the apex temperature Tmsh of the melting peak on the higher temperature side of the polypropylene-based resin (S) is -10°C or more and 15°C or less, a ratio [ΔHh / ΔHt] of the heat of fusion (ΔHh) of a melting peak on the high temperature side of the polypropylene resin (S) to the total heat of fusion (ΔHt) of the polypropylene resin (S) is 0.35 or more and 0.80 or less.

11. A method for producing expanded polypropylene-based resin particles, comprising the steps of: A step of granulating multilayer resin particles having a core layer made of a polypropylene-based resin (C) and a coating layer made of a polypropylene-based resin (S) that coats the core layer; dispersing the multilayered resin particles in a dispersion medium in a sealed container; impregnating the multilayered resin particles dispersed in the dispersion medium with a foaming agent; and a step of expanding the multilayer resin particles containing the foaming agent to produce expanded particles, The melting point Tmc of the polypropylene resin (C) is 125°C or higher and 170°C or lower, (1) The polypropylene resin (S) has a melting peak on the low temperature side and a melting peak on the high temperature side in a second DSC curve obtained by heat flux differential scanning calorimetry, in which the polypropylene resin (S) is heated from 30° C. to 200° C. at a heating rate of 10° C. / min, then cooled from 200° C. to 30° C. at a cooling rate of 10° C. / min, and heated again from 30° C. to 200° C. at a heating rate of 10° C. / min. (2) The apex temperature (Tmsl) of the melting peak on the low temperature side of the polypropylene-based resin (S) is lower than the melting point (Tmc) of the polypropylene-based resin (C); (3) The polypropylene resin (S) has a melting peak on the high temperature side (Tmsh) of 138° C. or higher; (4) The difference [Tmc-Tmsh] between the melting point Tmc of the polypropylene-based resin (C) and the apex temperature Tmsh of the melting peak on the higher temperature side of the polypropylene-based resin (S) is -10°C or more and 15°C or less, The method for producing expanded polypropylene resin beads, wherein the difference [Tmsh - Tmsl] between the apex temperature Tmsh of the higher melting peak and the apex temperature Tmsl of the lower melting peak of the polypropylene resin (S) is 10°C or more and 20°C or less.

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